Transparent Strip Around Border Of Render Preview
When you did ctl + b and drew the box to render you only selected that part that you see rendered. There is more to the scene but you would need to redraw your box (ctrl + B) around the whole box outlined in red (the render view). 

To recap: 

Just ctrl + b around the entire red square and you should be fine. 

The ctrl + b is just for rendering a certain part of your scene so you do have to waste time rendering everything.